The present invention relates generally to microelectronics circuits packaging and, more particularly, to a method for testing the integrity of the protective encapsulation of such packages.
Glassivation is a process by which a protective layer of a glass-like substance is coated over the die surface of an integrated circuit (IC). One purpose of this protective layer is to safeguard the active circuitry thereunder from conductive particles, contaminants or the corrosive effects of water vapor. The presence of any one of these impurities may significantly affect the electrical characteristics of the circuit.
In the past, it was observed that the failure of many integrated circuits was due to corrosion of the thin film nichrome resistors or to corrosion of the aluminum used in the metallization layer for interconnects. The cause of this corrosion was traced to inadequate glassivation or defects therein, and to the resulting moisture trapped within the package cavity. The solution to this failure problem was to improve the glassivation layer and to develop quality assurance testing to check the integrity of this layer. The currently used testing methods, such as nichrome and aluminum metal etch tests, which are described in greater detail in later paragraphs, are somewhat ambiguous in the fault detection process, relatively expensive to perform, time-consuming and, without proper handling, potentially hazardous to the health of the test operator.
The currently used glassivation integrity test procedure requires two acid-based etching solutions. A first solution, used for etching aluminum metal, comprises phosphoric acid (80% by volume), nitric acid (5%), acetic acid (5%) and deionized water (10%). The second solution, used for etching nichrome, comprises ceric sulfate (6 grams per 100 ml of solution), nitric acid (10% by volume) and deionized water (90%). Both solutions must be heated so that the etching process will occur at an increased rate, and the nichrome-etching solution must be constantly stirred to keep it well mixed. These solutions are relatively expensive, they require careful handling during use, and they must be disposed of in accordance with standards for hazardous waste material.
In accordance with a currently used test procedure, the part to be tested is inspected under a high-powered microscope for any gross mechanical damage. The part is then dipped into a heated acid solution, as described above, for a minimum of 15 seconds. The part is then thoroughly cleaned and dried, and again is inspected carefully under the high-powered microscope. In accordance with a capillary wicking effect, the acid will seep into any pinholes or cracks that exist in the glassivation layer, and corrode or dissolve any exposed circuitry, leaving a void which is visible under a high-powered microscope. Visible voids may then be photographed for permanent documentation. The capillary wicking effect is dependent on the surface tension of the applied liquid and the width of the crack. Heating of the acid solution does not significantly influence capillary wicking.
When etching of the metallization occurs as a result of a defect in the glassivation layer, there are two types of oxidation which can take place in the defective area. The first and most common reaction occurs when the acid enters the defective area and oxidizes the metal region it contacts. The by-product of this reaction exits through the defective region and flows out into the acid solution, thereby adding contaminants to the testing solution.
The second type of reaction is a limited diffused reaction. This is caused primarily by the shape of the void and the type of chemical reaction which occurs when the acid is exposed to metallization. The oxidation by-product is trapped in the void, resulting in a build-up which covers up the defective region. This results in a healing process in which the acid solution can no longer contact the defective area, thereby stopping the etching process. This type of by-product will appear as foreign material and can easily be misidentified as such during inspection, thereby imparting ambiguous results to the glassivation integrity test. This limited diffused reaction case is not encountered frequently; however, it is difficult to identify due to its appearance and lack of obvious voiding.
In view of the above, it is clear that there exists a pressing need to develop an encapsulation integrity test which avoids many or all of the disadvantages associated with the currently used process.